Several advantages exist for using copper metalization in integrated circuits, such as semiconductor devices. However, copper metalization may be more susceptible to oxidation under certain process conditions as compared to other metals, such as aluminum. Semiconductor devices often include at least two primary metal layers with interconnections between such layers. The first metal layer can be a so-called “metal 1” layer and the second can be a so-called “metal 2” layer.
The first metal layer may be formed on a substrate and covered by a dielectric material, such as silicon dioxide. An opening for an interconnect may then be formed through the dielectric material to expose the first metal layer. The opening may be formed by patterning a layer of photoresist deposited over the dielectric and etching portions of the dielectric material exposed through the photoresist. A common process for removing photoresist comprises ashing. Such removal of a photoresist exposes the first metal layer to the ashing conditions, potentially oxidizing the first metal layer. Copper is particularly susceptible to oxidation at high temperature processing, such as processing at 200° C. or higher.
One method for reducing oxidation of the first metal layer includes forming a layer of silicon nitride over the first metal layer prior to forming dielectric material over the first metal layer. The dielectric material is then processed as indicated above with formation of a photoresist, patterning of the photoresist, etching, and photoresist removal by ashing. However, after etching an opening for a conductive interconnect, a separate etch of the silicon nitride may be used to expose the first metal layer preparatory to forming a conductive interconnect to such layer. A high level of selectivity may often be provided for etching the silicon nitride compared to etching the dielectric material, such as silicon dioxide. The two-step etch process and highly selective etch of silicon nitride add a level of complexity to such processing that is undesirable.
Accordingly, new methods are desired for forming conductive connections between first and second metal layers in semiconductor devices that reduce oxidation of copper without introducing undue complexity to processing.